Pre-sintered blank for dental purposes

ABSTRACT

Pre-sintered blanks based on lithium disilicate glass ceramic are described which are suitable in particular for the preparation of dental restorations.

The invention relates to a pre-sintered blank for dental purposes based on lithium disilicate glass ceramic which blank is suitable in particular for the preparation of dental restorations.

Reports on the use of pre-sintered blanks in dentistry have already been made in the state of the art.

WO 2010/010087 describes porous silicate-ceramic shaped bodies which are processed to form veneers for dentistry. The shaped bodies are to have a particular density in order to prevent damage during the machining with milling or grinding systems, e.g. due to the material bursting, and to be suitable for the selected system.

U.S. Pat. No. 5,106,303 describes the preparation of tooth crowns and inlays by copy milling of compacted ceramic bodies which can optionally be pre-sintered. To achieve the desired geometry, the bodies are milled to an enlarged shape in order to take into consideration the shrinkage that occurs during the subsequent sintering to the desired high density. Aluminium oxide, which can optionally include strengthening additives, is used in particular as ceramic material.

U.S. Pat. No. 5,775,912 describes pre-sintered dental porcelain pellets, from which a tooth structure is milled by means of CAD/CAM systems. This tooth structure is embedded in embedding material, sintered and removed from the embedding material in order to produce the desired dental restoration. The dental porcelains used are glass ceramics based on leucite.

U.S. Pat. No. 6,354,836 discloses methods of manufacturing dental restorations using CAD/CAM methods. For this, unsintered or pre-sintered blocks of ceramic material and in particular aluminium oxide and zirconium oxide are used which result in high-strength dental restorations after milling to an enlarged shape followed by dense sintering. However, it is considered to be essential that the temperature differences in the sintering furnace used are smaller than 10° C. in order to ensure that variations in the finally achieved dimensions of the restorations are small.

With the known pre-sintered blanks, the shrinkage occurring during the dense sintering and thus the enlargement factor to be applied depends to a great extent on the pre-sintering temperature applied. Even small variations, such as can occur as a result of an inhomogeneous temperature distribution in the sintering furnace, result in different shrinkages during the dense sintering. However, these shrinkages do not allow the desired small tolerances in the dimensions of the produced dental restoration.

The object of the invention is therefore to provide pre-sintered blanks which avoid these disadvantages and are therefore less susceptible to variations in the sintering temperature applied for their preparation. Likewise, these blanks should be able to be shaped easily by means of customary grinding and milling processes to form dental restorations with the desired geometry, without liquid needing to be supplied during these processes. Furthermore, these blanks should be able to be processed by dense sintering to form high-strength and optically very attractive dental restorations.

This object is achieved by the pre-sintered blank according to claims 1 to 9. Another subject of the invention is the process for the preparation of the blank according to claims 10 and 11, the process for the preparation of dental restorations according to claims 12 to 15 as well as the use of the blank according to claim 16.

The pre-sintered blank according to the invention for dental purposes is characterized in that it

is based on lithium disilicate glass ceramic and has a relative density of from 60 to 90%, in particular 62 to 88% and preferably 65 to 87%, relative to the true density of the glass ceramic.

The relative density is the ratio of the density of the pre-sintered blank to the true density of the glass ceramic.

The density of the pre-sintered blank is determined by weighing it and ascertaining its volume geometrically. The density is then calculated according to the known formula

density=mass/volume.

The true density of the glass ceramic is determined by grinding the pre-sintered blank to a powder with an average particle size of from 10 to 30 μm, in particular of 20 μm, relative to the number of particles and ascertaining the density of the powder by means of a pycnometer. The determination of the particle size was carried out by means of laser diffraction in accordance with ISO 13320 (2009) with the CILAS® Particle Size Analyzer 1064 from Quantachrome GmbH & Co. KG.

It has surprisingly been found out that not only can the blank according to the invention be machined dry in a simple way, but it can also be prepared at significantly different pre-sintering temperatures, without this resulting in a substantial change in the shrinkage which occurs during a subsequent dense sintering. The enlargement factor taking into consideration the shrinkage that occurs can thus be determined very precisely. These advantageous properties are clearly to be attributed to the particular behaviour of lithium disilicate glass ceramic which was pre-sintered to the relative densities given above.

It is further preferred that the blank consists substantially of lithium disilicate glass ceramic. Particularly preferably, the blank consists of lithium disilicate glass ceramic.

The glass ceramic includes lithium disilicate as main crystal phase in a preferred embodiment. The term “main crystal phase” denotes the crystal phase which has the highest proportion by volume compared with other crystal phases. In particular the glass ceramic contains more than 10 vol.-%, preferably more than 20 vol.-% and particularly preferably more than 30 vol.-% lithium disilicate crystals, relative to the total glass ceramic.

The lithium disilicate glass ceramic contains SiO₂ and Li₂O, preferably in a molar ratio in the range of from 1.75 to 3.0, in particular 1.8 to 2.6.

In a further preferred embodiment, the lithium disilicate glass ceramic contains at least one of the following components:

Component wt.-% SiO₂ 50.0 to 80.0 Li₂O  6.0 to 20.0 Me (I) ₂O 0 to 10.0, in particular 0.1 to 10.0 Me (II) O 0 to 12.0, in particular 0.1 to 12.0 Me (III) ₂O₃ 0 to 8.0, in particular 0.1 to 8.0 Me (IV) O₂ 0 to 8.0, in particular 0.1 to 8.0 Me (V) ₂O₅ 0 to 8.0, in particular 0.1 to 8.0 Me (VI) O₃ 0 to 8.0, in particular 0.1 to 8.0 nucleating agent 0 to 8.0, in particular 0.1 to 8.0

-   -   wherein     -   Me(I)₂O is selected from Na₂O, K₂O, Rb₂O, Cs₂O or mixtures         thereof,     -   Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixtures         thereof,     -   Me(III)₂O₃ is selected from Al₂O₃, La₂O₃, Bi₂O₃, Y₂O₃, Yb₂O₃ and         mixtures thereof,     -   Me(IV)O₂ is selected from ZrO₂, TiO₂, SnO₂, GeO₂ and mixtures         thereof,     -   Me (V)₂O₅ is selected from Ta₂O₅, Nb₂O₅, V₂O₅ and mixtures         thereof,     -   Me(VI)O₃ is selected from WO₃, MoO₃ and mixtures thereof, and     -   nucleating agent is selected from P₂O₅, metals and mixtures         thereof.

Na₂O and K₂O are preferred as oxides of monovalent elements Me(I)₂O.

CaO, MgO, SrO and ZnO are preferred as oxides of divalent elements Me(II)O.

Al₂O₃, La₂O₃ and Y₂O₃ are preferred as oxides of trivalent elements Me(III)₂O₃.

ZrO₂, TiO₂ and GeO₂ are preferred as oxides of tetravalent elements Me(IV)O₂.

Ta₂O₅ and Nb₂O₅ are preferred as oxides of pentavalent elements Me(V)₂O₅.

WO₃ and MoO₃ are preferred as oxides of hexavalent elements Me(VI)O₃.

P₂O₅ is preferred as nucleating agent.

The lithium disilicate glass ceramic preferably contains colorants and/or fluorescent agents.

Examples of colorants and fluorescent agents are inorganic pigments and/or oxides of d- and f-elements, such as the oxides of Ti, V, Sc, Mn, Fe, Co, Ta, W, Ce, Pr, Nd, Tb, Er, Dy, Gd, Eu and Yb. Metal colloids, e.g. of Ag, Au and Pd, can also be used as colorants and in addition can also act as nucleating agents. These metal colloids can be formed e.g. by reduction of corresponding oxides, chlorides or nitrates during the melting and crystallization processes. For example, doped spinels, zircon silicate, stannates, doped corundum and/or doped ZrO₂ are used as inorganic pigments.

The blank according to the invention preferably has at least two areas, in particular layers, which differ in terms of their coloration or translucence. The blank preferably has at least 3 and up to 10, particularly preferably at least 3 and up to 8, and even more preferably at least 4 and up to 6 areas, in particular layers, differing in coloration or translucence. The imitation of natural tooth material is very successful precisely because of the presence of several differently coloured areas, in particular layers. It is also possible that at least one of the areas or of the layers has a colour gradient to ensure a continuous colour transition.

In a further preferred embodiment, the blank according to the invention has a holder for securing it in a processing device. In another preferred embodiment, the blank according to the invention has an interface for connection to a dental implant.

The holder allows the blank to be secured in a processing device, such as in particular a milling or grinding device. The holder is usually in the form of a pin and preferably consists of metal or plastic.

The interface ensures a connection between an implant and the dental restoration fitted thereon, such as in particular an abutment crown, which has been obtained by machining and dense sintering of the blank. This connection is preferably rotationally fixed. The interface is present in particular in the form of a recess, such as a bore. The specific geometry of the interface is usually chosen depending on the implant system used in each case.

The invention also relates to a process for the preparation of the blank according to the invention, in which

-   (a) lithium silicate glass in powder or granulate form is pressed to     form a glass blank, -   (b) the glass blank is heat-treated in order to prepare a     pre-sintered blank based on lithium disilicate glass ceramic,     wherein the temperature of the heat treatment     -   (i) is at least 500° C., in particular at least 540° C. and         preferably at least 580° C., and     -   (ii) lies in a range which extends over at least 30K, in         particular at least 50K and preferably at least 70K and in which         the relative density varies by less than 2.5%, in particular         less than 2.0% and preferably less than 1.5%.

In stage (a), lithium silicate glass in powder or granulate form is pressed to form a glass blank.

The lithium silicate glass employed is usually prepared by melting a mixture of suitable starting materials, such as carbonates, oxides, phosphates and fluorides, for 2 to 10 h at temperatures of in particular from 1300 to 1600° C. To achieve a particularly high homogeneity, the obtained glass melt is poured into water in order to form a glass granulate, and the obtained granulate is then melted again.

The granulate is then comminuted to the desired particle size and in particular ground to powder with an average particle size of <100 μm, relative to the number of particles.

The granulate or powder is then, optionally together with added pressing auxiliaries or binders, usually placed in a compression mould and pressed to form a glass blank. The pressure applied lies in particular in the range of from 20 to 200 MPa. Uniaxial presses are preferably used for the pressing. The pressing can in particular also be isostatic pressing, preferably cold isostatic pressing.

Through the use of glass powders or glass granulates with different coloration or translucence, glass blanks can be produced which have differently coloured or differently translucent areas and in particular layers. For example, differently coloured powders or granulates can be arranged on top of one another in a compression mould, with the result that a multi-coloured glass blank is produced. The multiple colours make it possible to a great extent to give the finally prepared dental restorations the appearance of natural tooth material.

In stage (b), the obtained uni- or multi-coloured glass blank is subjected to a heat treatment in order to bring about the controlled crystallization of lithium disilicate and thus the formation of a lithium disilicate glass ceramic as well as the pre-sintering. The heat treatment takes place in particular at a temperature of from 500° C. to 900° C., preferably from 540 to 900° C. and particularly preferably from 580 to 900° C. The heat treatment is carried out in particular for a period of from 2 to 120 min, preferably 5 to 60 min and particularly preferably 10 to 30 min.

The temperature range (b)(ii) describes a range in which, despite a change in temperature, the relative density hardly changes. This range is therefore also called “plateau” in the following. The variation in the relative density possible in this range is calculated in % from the maximum and minimum value of the relative density in the range by

(maximum value−minimum value)/maximum value×100

It has surprisingly been shown that during its production and pre-sintering in particular temperature ranges lithium disilicate glass ceramics display essentially no change in the relative density and thus in the linear shrinkage and the enlargement factor during the dense sintering. These ranges are recognizable as “plateaus” in the graphic representation of relative density, linear shrinkage or enlargement factor against the temperature. Accordingly, properties of the blank that are important for the accuracy of fit of the later dental restoration are essentially not dependent on the temperature in this range. The result of this is the important practical advantage that the blank tends to be unsusceptible e.g. to temperature fluctuations or temperature gradients in the sintering furnace, as long as the temperature is in the “plateau” range.

According to the invention, therefore, pre-sintered blanks which are obtainable by the process according to the invention are particularly preferred.

Particularly preferred are blanks according to the invention which have a relative density which results when

-   (a) powder of a corresponding starting glass with an average     particle size of <100 μm, relative to the number of particles, is     uniaxially or isostatically pressed at a pressure of from 20 to 200     MPa, preferably 40 to 120 MPa and particularly preferably 50 to 100     MPa and -   (b) the obtained glass powder green compact is heat-treated for to     120 min, preferably 5 to 60 min and particularly preferably 10 to 30     min at a temperature which     -   (i) is at least 500° C., in particular at least 540° C. and         preferably at least 580° C., and     -   (ii) lies in a range which extends over at least 30K, in         particular at least 50K and preferably at least 70K and in which         the relative density varies by less than 2.5%, in particular         less than 2.0% and preferably less than 1.5%.

FIG. 2 illustrates the phases usually passed through during heat treatment of a glass powder green compact by plotting the enlargement factor against the temperature for a green compact with a composition according to Example 2. In Phase I, up to about 500° C., the heating and the removal of any binder present take place. In Phase II, from about 500 to 600° C., sintering and crystallization take place, and in Phase III, the plateau, from about 600 to about 850° C., there is a pre-sintered blank according to the invention based on lithium disilicate glass ceramic. Then, in Phase IV, from about 850 to about 950° C., the dense sintering of the blank takes place.

The pre-sintered blank according to the invention is preferably present in the form of blocks, disks or cylinders. In these forms, a further processing to form the desired dental restorations is particularly easy.

The pre-sintered blank is further processed in particular to form dental restorations. The invention therefore also relates to a process for the preparation of dental restorations, in which

(i) the pre-sintered blank according to the invention based on lithium disilicate glass ceramic is shaped by machining to form a precursor of the dental restoration, (ii) the precursor is substantially dense sintered in order to produce the dental restoration, and (iii) optionally the surface of the dental restoration is provided with a finish.

In stage (i), the machining is usually carried out by material removal processes and in particular by milling and/or grinding. It is preferred that the machining is carried out with computer-controlled milling and/or grinding devices. Particularly preferably, the machining is carried out as part of a CAD/CAM process.

The blank according to the invention can be machined very easily in particular because it is open-pored and has low strength. It is particularly advantageous that it is not necessary to use liquids during the grinding or milling. In contrast to this, so-called wet-grinding processes are often necessary for conventional blanks.

The machining is usually carried out in such a way that the obtained precursor represents an enlarged form of the desired dental restoration. The shrinkage occurring during the subsequent dense sintering is thereby taken into consideration. The blank according to the invention has the particular advantage that the enlargement factor to be applied to it can be determined very precisely. The enlargement factor is the factor by which the precursor has to be ground or milled enlarged out of the pre-sintered blank in order that after the dense sintering the obtained dental restoration has the desired dimensions.

The enlargement factor F_(v), the relative density ρ_(r) and the remaining linear shrinkage S can be converted into each other as follows:

S=1−ρ_(r) ^(1/3)

F _(v)=1/(1−S)

In a preferred embodiment, the blank produced according to the above-described process according to the invention is used as pre-sintered blank.

In stage (ii) the obtained precursor is substantially dense-sintered in order to produce the dental restoration with the desired geometry.

For the dense sintering, the precursor is preferably heat-treated at a temperature of from 700 to 1000° C. The heat treatment usually takes place for a period of from 2 to 30 min.

After the dense sintering, there is a dental restoration based on lithium disilicate glass ceramic in which lithium disilicate preferably forms the main crystal phase. This lithium disilicate glass ceramic has excellent optical and mechanical properties as well as a high chemical stability. Dental restorations which meet high demands can thus be prepared with the process according to the invention.

The dental restorations are preferably selected from crowns, abutments, abutment crowns, inlays, onlays, veneers, shells and bridges as well as overstructures for multi-part restoration frames which can consist e.g. of oxide ceramic, metals or dental alloys.

It can be advantageous for the dense sintering that the precursor of the dental restoration is supported in order to avoid a distortion. It is preferred that the support consists of the same material as the precursor and hence shows the same shrinkage upon sintering. The support can be in form of for example a supporting structure or supporting mould which in terms of their geometry are adapted to the precursor.

In the optional stage (iii), the surface of the dental restoration can also be provided with a finish. It is possible in particular to also carry out a glazing firing at a temperature of from 650 to 900° C. or to polish the restoration.

Because of the described properties of the pre-sintered blank according to the invention, it is suitable in particular for producing dental restorations. The invention therefore also relates to the use of the blank to prepare dental restorations and in particular crowns, abutments, abutment crowns, inlays, onlays, veneers, shells and bridges as well as overstructures.

The average particle sizes given, relative to the number of particles, were determined at room temperature by laser diffraction with the CILAS® Particle Size Analyzer 1064 from Quantachrome GmbH & Co. KG in accordance with ISO 13320 (2009).

The invention is explained in more detail below by means of examples.

EXAMPLES Examples 1 to 4

A total of 4 glass ceramics with lithium disilicate as main crystal phase with the composition given in Table I were prepared by melting corresponding starting glasses and then, by heat treatment, presintering glass powder blanks produced from them, and crystallizing them in a controlled manner.

For this, the starting glasses on a scale of 100 to 200 g were first melted from customary raw materials at 1400 to 1500° C., wherein the melting could be carried out very easily without formation of bubbles or streaks. By pouring the starting glasses into water, glass frits were prepared which were then melted a second time at 1450 to 1550° C. for 1 to 3 h for homogenization.

The obtained glass melts were then cooled to 1400° C. and converted to fine-particle granulates by pouring into water. The granulates were dried and ground to powder with an average particle size of <100 μm, relative to the number of particles. These powders were moistened with some water and pressed to form powder green compacts at a pressing pressure of from 20 to 200 MPa.

The powder green compacts were then heat-treated for 2 to 120 min at a temperature which lies in the range given as plateau in Table I for the respective composition. After this heat treatment, blanks according to the invention were present which were pre-sintered and based on lithium disilicate glass ceramic.

TABLE I Example 1 2 3 4 Component wt.-% wt.-% wt.-% wt.-% SiO₂ 67.5 75.2 78.4 69.4 Li₂O 14.9 15.6 16.3 19.7 P₂O₅ 4.3 — 3.3 3.4 K₂O 4.2 — — — MgO 0.7 — — — SrO — 4.1 — — ZnO 4.8 — — — Al₂O₃ — 3.6 — 3.5 La₂O₃ 1.0 — — — Er₂O₃ — 0.25 — — CeO₂ 2.0 1.0 — — SnO₂ — — 2.0 — Nb₂O₅ — — — — V₂O₅ 0.1 — — — MoO₃ — — — 4.0 Tb₄O₇ 0.5 — — — Main crystal phase LS2 LS2 LS2 LS2 Plateau (° C.) 600- 590- 600- 600- 800 860 900 900 LS2 lithium disilicate

Example 5 Examination of Sintering Behaviour of the Composition According to Example 1

A glass with the composition according to Example 1 was melted and ground to a glass powder with an average particle size of less than 50 μm, relative to the number of particles. This glass powder was pressed to form cylinders. The sintering behaviour of these cylindrical blanks was examined by heat-treating them at different temperatures in a furnace of the Programat® P500 type from Ivoclar Vivadent AG. In each case a heating rate of 20° C./min and a holding time of 2 min at the respective temperature were used. After that the blanks were cooled to room temperature and the relative density of the blanks was then determined in each case in relation to the true density of the glass ceramic. The remaining linear shrinkage as well as the enlargement factor were calculated from the relative density.

The results for temperatures in the range of from 25 to 900° C. are shown in the following Table II. A pre-sintered lithium disilicate glass ceramic blank according to the invention with a relative density of from 69 to 70% was present at between 600 and 800° C.

TABLE II Temperature [° C.] 25 450 500 550 600 650 700 750 800 850 900 Diameter [mm] 16.10 16.09 16.07 15.47 14.55 14.59 14.61 14.56 14.48 13.39 13.12 Height [mm] 15.16 15.14 15.05 14.91 13.81 13.80 13.83 13.88 13.92 12.65 12.03 Volume [cm3] 3.09 3.08 3.05 2.80 2.30 2.31 2.32 2.31 2.29 1.78 1.63 Mass [g] 4.00 3.98 3.99 4.05 3.98 3.98 3.97 3.99 4.01 4.03 3.98 Density [g/cm3] 1.30 1.29 1.31 1.45 1.74 1.73 1.71 1.73 1.75 2.26 2.45 Relative density [%] 52 52 52 58 69 69 69 69 70 91 98 Linear shrinkage [%] 19.6 19.5 19.2 17.3 11.4 11.5 11.6 11.6 11.5 3.5 0.0 Enlargement factor 1.24 1.24 1.24 1.21 1.13 1.13 1.13 1.13 1.13 1.04 1.00

In FIG. 1, the calculated enlargement factor is plotted against the temperature applied. It can be seen from this that the enlargement factor surprisingly remains substantially constant in the range of from 600 to 800° C. and the curve forms a plateau. Thus, when a heat treatment is applied in this range, a blank according to the invention can be produced for which a very precise specification of the enlargement factor to be chosen is possible.

The same process for determining this range (“plateau”) was also used for the other compositions given in Table I.

Example 6 Examination of Sintering Behaviour of the Composition According to Example 2

The sintering behaviour of the composition according to Example 2 was examined analogously to Example 5. The values obtained for the relative density, the remaining linear shrinkage and the enlargement factor are listed in Table III.

TABLE III Temperature [° C.] 30 450 500 550 600 650 700 750 800 850 900 950 Linear shrinkage [%] 14.05 14.10 14.30 12.80 11.45 11.45 11.55 11.45 11.40 11.45 9.95 0.00 Enlargement factor 1.163 1.164 1.167 1.147 1.129 1.129 1.131 1.129 1.129 1.129 1.110 1.000 Relative density [%] 63 63 63 66 69 69 69 69 70 70 73 100

The enlargement factor was plotted against the temperature and the curve obtained is shown in FIG. 2. It can be seen from this that the plateau for the examined lithium disilicate glass ceramic is in a range of about 600 to about 850° C. A pre-sintered lithium disilicate glass ceramic blank according to the invention with a relative density of from 69 to 70% is present in this range. 

1. Pre-sintered blank for dental purposes based on lithium disilicate glass ceramic, wherein the blank has a relative density of from 60 to 90%, relative to the true density of the glass ceramic.
 2. Pre-sintered blank according to claim 1, which consists substantially of lithium disilicate glass ceramic.
 3. Pre-sintered blank according to claim 1, wherein the glass ceramic includes lithium disilicate as main crystal phase and contains more than 10 vol. % lithium disilicate crystals.
 4. Pre-sintered blank according to claim 1, wherein the lithium disilicate glass ceramic contains at least one of the following components: Component wt.-% SiO₂ 50.0 to 80.0 Li₂O  6.0 to 20.0 Me(I)₂O   0 to 10.0 Me(II)O   0 to 12.0 Me(III)₂O₃   0 to 8.0 Me(IV)O₂   0 to 8.0 Me(V)₂O₅   0 to 8.0 Me(VI)O₃   0 to 8.0 nucleating agent   0 to 8.0

wherein Me(I)₂O is selected from Na₂O, K₂O, Rb₂O, Cs₂O or mixtures thereof, Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixtures thereof, Me(III)₂O₃ is selected from Al₂O₃, La₂O₃, Bi₂O₃, Y₂O₃, Yb₂O₃ and mixtures thereof, Me(IV)O₂ is selected from ZrO₂, TiO₂, SnO₂, GeO₂ and mixtures thereof, Me(V)₂O₅ is selected from Ta₂O₅, Nb₂O₅ and mixtures thereof, Me(VI)O₃ is selected from WO₃, MoO₃ and mixtures thereof, and nucleating agent is selected from P₂O₅, metals and mixtures thereof.
 5. Pre-sintered blank according to claim 1, which has at least two areas which differ by their coloration or translucence.
 6. Pre-sintered blank according to claim 1, which has a holder for a processing device.
 7. Pre-sintered blank according to claim 1, which has an interface, in particular in the form of a recess, for connection to a dental implant.
 8. Pre-sintered blank according to claim 1, which is obtainable by the process in which (a) lithium silicate glass in powder or granulate form is pressed to form a glass blank, (b) the glass blank is heat-treated in order to prepare a pre-sintered blank based on lithium disilicate glass ceramic, wherein the temperature of the heat treatment (i) is at least 500° C., and (ii) lies in a range which extends over at least 30K, and in which the relative density varies by less than 2.5%.
 9. Pre-sintered blank according to claim 1, which has a relative density which results when (a) powder of a corresponding starting glass with an average particle size of <100 μm, relative to the number of particles, is uniaxially or isostatically pressed at a pressure of from 20 MPa to 200 MPa, preferably 40 to 120 MPa and particularly preferably 50 to 100 MPa and (b) the obtained glass powder green compact is heat-treated for 2 to 120 min, preferably 5 to 60 min and particularly preferably 10 to 30 min at a temperature which (i) is at least 500° C., in particular at least 540° C. and preferably at least 580° C., and (ii) lies in a range which extends over at least 30K, in particular at least 50K and preferably at least 70K and in which the relative density varies by less than 2.5%, in particular less than 2.0% and preferably less than 1.5%.
 10. Process for the preparation of the blank according to claim 1, in which (a) lithium silicate glass in powder or granulate form is pressed to form a glass blank, (b) the glass blank is heat-treated in order to prepare a pre-sintered blank based on lithium disilicate glass ceramic, wherein the temperature of the heat treatment (i) is at least 500° C., and (ii) lies in a range which extends over at least 30K and in which the relative density varies by less than 2.5%.
 11. Process according to claim 10, in which in step (a) at least two lithium silicate glasses are used which differ in terms of their coloration or translucence.
 12. Process for the preparation of dental restorations, in which (i) the pre-sintered blank based on lithium disilicate glass ceramic according to claim 1 is shaped by machining to form a precursor of the dental restoration, (ii) the precursor is substantially dense-sintered in order to produce the dental restoration, and (iii) optionally the surface of the dental restoration is provided with a finish.
 13. Process according to claim 12, in which the machining is carried out with computer-controlled milling and/or grinding devices.
 14. Process according to claim 12, in which (a) lithium silicate glass in powder or granulate form is pressed to form a glass blank, (b) the glass blank is heat-treated in order to prepare a pre-sintered blank based on lithium disilicate glass ceramic, wherein the temperature of the heat treatment (i) is at least 500° C., and (ii) lies in a range which extends over at least 30K and in which the relative density varies by less than 2.5% and is carried out in order to obtain the pre-sintered blank based on lithium disilicate glass ceramic.
 15. Process according to claim 12, in which the dental restorations are selected from crowns, abutments, abutment crowns, inlays, onlays, veneers, shells, bridges and overstructures.
 16. A method of using the blank according to claim 1 to prepare dental restorations comprising crowns, abutments, abutment crowns, inlays, onlays, veneers, shells, bridges and overstructures.
 17. Pre-sintered blank according to claim 1, wherein the blank has a relative density of from 62 to 88%, relative to the true density of the glass ceramic.
 18. Pre-sintered blank according to claim 1, wherein the blank has a relative density of from 65 to 87%, relative to the true density of the glass ceramic.
 19. Pre-sintered blank according to claim 3, wherein the glass ceramic contains more than 20 vol.-% lithium disilicate crystals.
 20. Pre-sintered blank according to claim 3, wherein the glass ceramic contains more than 30 vol.-% lithium disilicate crystals.
 21. Pre-sintered blank according to claim 1, wherein the lithium disilicate glass ceramic contains at least one of the following components: Component wt.-% SiO₂ 50.0 to 80.0 Li₂O  6.0 to 20.0 Me(I)₂O  0.1 to 10.0 Me(II)O  0.1 to 12.0 Me(III)₂O₃  0.1 to 8.0 Me(IV)O₂  0.1 to 8.0 Me(V)₂O₅  0.1 to 8.0 Me(VI)O₃  0.1 to 8.0 nucleating agent  0.1 to 8.0,

wherein Me(I)₂O is selected from Na₂O, K₂O, Rb₂O, Cs₂O or mixtures thereof, Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixtures thereof, Me(III)₂O₃ is selected from Al₂O₃, La₂O₃, Bi₂O₃, Y₂O₃, Yb₂O₃ and mixtures thereof, Me(IV)O₂ is selected from ZrO₂, TiO₂, SnO₂, GeO₂ and mixtures thereof, Me(V)₂O₅ is selected from Ta₂O₅, Nb₂O₅ and mixtures thereof, Me(VI)O₃ is selected from WO₃, MoO₃ and mixtures thereof, and nucleating agent is selected from P₂O₅, metals and mixtures thereof.
 22. Pre-sintered blank according to claim 5, wherein the at least two areas comprise layers.
 23. Pre-sintered blank according to claim 7, wherein the interface comprises a recess for connection to the dental implant.
 24. Pre-sintered blank according to claim 9, which has a relative density which results when (a) powder of a corresponding starting glass with an average particle size of <100 μm, relative to the number of particles, is uniaxially or isostatically pressed at a pressure of from 40 to 120 MPa and particularly preferably 50 to 100 MPa and (b) the obtained glass powder green compact is heat-treated for 5 to 60 min and particularly preferably 10 to 30 min at a temperature which (i) is at least 540° C. and preferably at least 580° C., and (ii) lies in a range which extends over at least 50K and preferably at least 70K and in which the relative density varies by less than 2.0% and preferably less than 1.5%.
 25. Pre-sintered blank according to claim 9, which has a relative density which results when (a) powder of a corresponding starting glass with an average particle size of <100 μm, relative to the number of particles, is uniaxially or isostatically pressed at a pressure of from 50 to 100 MPa and (b) the obtained glass powder green compact is heat-treated for 10 to 30 min at a temperature which (i) is at least 580° C., and (ii) lies in a range which extends over at least 70K and in which the relative density varies by less than 1.5%.
 26. Process for the preparation of the blank according to claim 10, in which (a) lithium silicate glass in powder or granulate form is pressed to form a glass blank, (b) the glass blank is heat-treated in order to prepare a pre-sintered blank based on lithium disilicate glass ceramic, wherein the temperature of the heat treatment (i) is at least 540° C. and preferably at least 580° C., and (ii) lies in a range which extends over at least 50K and preferably at least 70K and in which the relative density varies by less than 2.0% and preferably less than 1.5%.
 27. Process for the preparation of the blank according to claim 10, in which (a) lithium silicate glass in powder or granulate form is pressed to form a glass blank, (b) the glass blank is heat-treated in order to prepare a pre-sintered blank based on lithium disilicate glass ceramic, wherein the temperature of the heat treatment (i) is at least 580° C., and (ii) lies in a range which extends over at least 70K and in which the relative density varies by less than 1.5%. 